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Questions about designing and routing with RS485

fastworld555 , 01-12-2024, 07:58 AM
I am using the SP3485 to interface the MCU's UART to RS485 using a SP3485 transceiver. the datasheet of SP3485 states that the driver outputs of SP3485 are differential outputs meeting with the RS485 and RS422 standards, while the SP3485 receiver has differential inputs. Does this mean that the traces for pins A and B should both be set as differential pairs in the EDA software? I've received mixed responses on this some saying that pins A and B should be set as differential pairs while others saying it's not necessary. I've also looked through some designs on the internet and none of the designs I've seen use differential pairs either. Maybe it doesn't really matter if the traces of A and B are set as differential pairs or not because the speed is slow?

Also, is it good practice to set the differential pair as 120 Ohms impedance if it's necessary to use differential pair routing?
Robert Feranec , 01-12-2024, 03:58 PM
you can route it as diff pair, but, usually transceivers should be placed as close as possible to the connector, so there may not be much place to route it is a diff pair.
QDrives , 01-12-2024, 05:03 PM
I would just add this: https://www.youtube.com/watch?v=pXSK9KlNsW4&pp=ygUPcm9iZXJ0IGZlcmFuZWMg
fastworld555 , 01-15-2024, 01:27 AM
HI Robert, this makes sense to me. Thank you very much for your help and clarification!
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